Electronic tube switching system



Nov. 1, 1955 A. DAVISON ET AL. 2,722,567

ELECTRONIC TUBE SWITCHING SYSTEM Filed Jan. 8, 1952 6 Sheets-Sheet lLFDR F5 1 Z 3 4 1 2 l w l v v L A AMPi TCL. A

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3 LUMC CMC INVENTORE A T TO/PNEYS Nov. 1, 1955 A. DAVISON ET ALELECTRONIC TUBE SWITCHING SYSTEM 6 Sheets-Sheet 2 Filed Jan. 8, 1952ALA/V DA V/SD/V /PO/VALD THREADGULD B y W X" JZm'fZ.

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ELECTRONIC TUBE SWITCHING SYSTEM 6 Sheets-Sheet 3 UMAi UMB1 UMAZ UMBZTSC Z2 MR3O MR31 23 MR32 MR33 21 Nov. 1, 1955 A. DAVISON ET AL 2,567

ELECTRONIC TUBE SWITCHING SYSTEM Filed Jan. 8, 1952 e Sheets-Sheet 5 INVE N TURS A LAN DA V/S UN fiflA/ALD THREADGULD v 354W wm'a.

A TT'UE'NE YS Nov. 1, 1955 A. DAVISON ET AL 2,722,567

ELECTRONIC TUBE SWITCHING SYSTEM Filed Jan. 8, 1952 6 Sheets-Sheet 6+80V N43 N44 2 3 W53 .2 Z -24v -24v T /NV'NTOES 41M 04 v/so/v 490mmTH/PE4DGULD 119.5. WMM

A TTURNE Y5 United States Patent 2,722,567 ELECTRONIC TUBE SWITCHINGSYSTEM Alan Davison and Ronald Threadgold, Liverpool, England, assiglors to Automatic Telephone & Electric Company Limited, Liverpool,England, a British com- P y Application January 8, 1952, Serial No.265,486 37 Claims. (Cl. 179-18) The present invention relates totelecommunication or like systems and is more particularly concernedwith switching arrangements in which the usualelectromagnetically-operated devices are replaced by their electronicequivalents.

The invention is alsoconcerned with arrangements for testing for an idleoutlet and for determining whether a selected outlet is idle or busy.

According to one feature of the invention, in a switching device, a setof electronic tubes is provided as circuitclosing means forconversational and control paths.

According to another feature of the invention, in a coordinate switchingdevice for use in a telecommunication or like system, a set of dischargetubes is provided as circuit-closing means at each coordinateintersection.

According to a further feature of the invention, in a switchingarrangement comprising a plurality of sets of gas discharge pathsarranged in groups, each set being adapted to be struck to completeconnections between input and output paths two voltages are employed forcontrolling the striking of the discharge paths, one voltage beingapplied to all sets of discharge paths in a particular group while thesecond is applied to corresponding sets of discharge paths in all thegroups whereby one set of discharge paths only is struck in response tothe joint application of said two voltages.

According to another feature of the invention, in a switchingarrangement comprising a plurality of sets of gas discharge tubesarranged in groups, each set being adapted to be struck to completeconnections over conversational and control paths one voltage is appliedto corresponding electrodes of all sets of discharge tubes in aparticular group and a second voltage is applied to said correspondingelectrodes of corresponding sets of discharge tubes in all the groups,one set of discharge tubes only striking in response to the jointapplication of said two voltages.

According to a further feature of the invention, in a switchingarrangement for associating a calling line with subsequent equipment andcomprising a plurality of sets of gas discharge tubes arranged ingroups, each set being adapted to be struck to complete connections overconversational and control paths means for determining the number of thecalling line serve to apply a biasing voltage to correspondingelectrodes of corresponding sets of dis charge tubes in all the groupsas determined by the number of the calling line while arrangements areprovided for allotting each group of discharge tube sets to callinglines in turn by the application of a pulse to said correspondingelectrodes of all the sets of discharge tubes in a group whereby one setof tubes in the allotted group only strikes to extend the calling lineto subsequent equipment by the joint application to said correspondingelectrodes of said biasing voltage and said pulse.

According to yet another feature of the invention, in a switchingarrangement for completing connections to called lines and comprisingaplurality of sets of gas discharge tubes arranged in groups, each setbeing adapted to be struck to complete connections over conversationaland control paths means are provided for registering the penultimate andfinal digits of a called partys number and for extending a biasingvoltage to corresponding electrodes of all the sets of gas dischargetubes in one group as determined by one of said digits and for extendinga pulse to said corresponding electrodes of corresponding sets of gasdischarge tubes in all groups as determined by the other of said digitswhereby one set of tubes only strikes to complete the connection to thecalled line by the joint application to said corresponding electrodes ofsaid biasing voltage and said pulse.

According to a further feature of the invention, in a switchingarrangement comprising a plurality of sets of gas discharge tubesarranged in groups, each set being adapted to be struck to completeconnections over conversational and control paths means are provided forregistering a digit necessary for extending a connection over a desiredroute and for extending a biasing voltage to corresponding electrodes ofall the sets of gas discharge tubes in one group as determined by saiddigit in order to select said group while means are provided whichsubsequently become operative to deliver pulses successively to saidcorresponding electrodes of corresponding sets of gas discharge tubes inall groups until available conversational and control paths within theselected group are found whereupon the set of gas discharge tubesappertaining to said idle paths are struck by the joint application tosaid corresponding electrodes of said biasing voltage and said pulse.

According to still another feature of the invention, in a switchingarrangement comprising a plurality of sets of gas discharge tubes, eachset being adapted to be struck to complete connections between input andoutput circuits a voltage applied to an electrode of one tube of a setcauses said tube to strike if the outlet appertaining to said set isavailable thereby causing a voltage to be applied to similar electrodesof the remaining tubes of said set while if the outlet is unavailable,the striking of said first tube together with the application of saidsecond voltage to the remaining tubes is prevented.

According to a further feature of the invention, in an electronic relay,a thermionic tube to which an input signal is applied is arranged tocontrol the operation of a gas discharge tube the electrodes betweenwhich the discharge takes place being connected between the anode ofsaid thermionic tube and a source of negative potential in such a mannerthat said discharge tube strikes only when said thermionic tube is cutoff.

The invention will be better understood from the following descriptionof one embodiment taken in conjunction with the accompanying drawingscomprising Figs. l-7.

In the drawings,

Fig. 1 shows a block schematic of the system.

Figs. 2-6 show the detailed circuits of the system of Fig. 1 and shouldbe arranged as shown in Fig. 7.

It should be explained that the invention has been illustrated withreference to a P. A. X having a capacity of lines and employing theelectronic equivalent of finder switches and final selector switches. Inthe embodiment shown in Fig. 1, four line finder switches LFDRI, LRDR2,LRDR3, LFDR4 are employed, each having access to all the lines of thesystem. One line only is shown with its accompanying line circuit LCA. Acontrol and marking circuit CMC is provided in common to all lines forthe purpose of registering the digits of a calling line and forextending a marking to the line finders in accordance with theregistered digits to cause a connection to be completed between apreselected line finder and the calling line circuit. It will, ofcourse, be understood that in a system having a greater capacity than100 lines it may be necessary to provide more than one control andmarking circuit in which case an allotter would probably have to beprovided for allotting the circuits for use in turn. The line findersthemselves are allotted for use in turn by the allotter AL which isprovided in common to all the line finders and which serves to reset thecontrol and marking circuit CMC at the appropriate time over the resetlead RS.

The control and marking circuit CMC comprises a tens marking counterLTMC, a units marking counter LUMC and a sequence switch LSS. The tensand units marking counters take the form of electronic countersemploying cold cathode tubes arranged in a ring-often but obviouslyother forms of electronic counters could be used. The counters arecontinuously pulsed from a suitable source and when a line is in thecalling condition the counters are stopped in the appropriate positionto register the digits by a control over the leads TCL and UCL extendingthereto from the line circuits. The sequence switch also consists of acold cathode tube counter and is pulsed from the tens and units markingcounters.

The circuit-closing means in the conversational and control pathsconsists sets of electronic tubes preferably cold cathode tubes and asregards the finder switches the selection of the tubes as determined bythe tens and units digits registration is effected through a markingfield to which potentials are applied from the tens and units markingcounters.

The final selectors are controlled by a tens marking counter FTMC and aunits marking counter FUMC which respond to the digits dialled by thecalling party and are controlled in their operation by the sequenceswitch PS5. The marking counters FTMC and FUMC and the sequence switchFSS are similar to the corresponding equipment in the control andmarking circuit CMC but it will, of course, be understood that FTMC,FUMC and PS8 are provided individual to each final selector. Similarlythe circuit-closing means in the final selector conversational andcontrol paths consist of cold cathode tubes and when fired complete theconnection to the called line circuit,

Two final selectors PS1 and F52 only have been shown but it will beunderstood that if four line finders are used, four final selectors willbe necessary, each line finder being permanently associated with a finalselector preferably through an amplifier as shown in Fig. 1.

A description will now be given of the detailed circuit of the systemshown in Figs. 2-6.

When the calling subscriber lifts off his handset, the line is loopedand current flows through R and L10 in the line circuit LCA and negativepotential of approximately 30 volts appears across L10. This potentialis applied to the cathode of the gas discharge tube CCL2 for a purposewhich will be described later and also causes a current flow from earthat the lower end of the tens resistor R40 (Fig. 4) in the line findertens-marking counter LTMC, over the tens lead TCL, rectifier MR10 andresistor R11. There are ten resistors such as R40 each connected inparallel to lines having the same tens digit. The top end of eachresistor, such as R40, is connected via a resistor and gas dischargetube, such as R44 and N50 to the control electrode of one of the gasdischarge tubes in the counter. A continuously-operating pulse generatorPG10 feeds pulses over ten leads each of which is connected over acapacitor such as C31 to the right-hand end of the resistor such as R44.If none of the lines are in a calling condition, the junction of theresistors, such as R40 and R44 will be at earth potential so that thedischarge tubes, such as N50, will strike in response to pulses fromPG10. It will be understood that each neon tube N50 is individual to aparticular tens digit and is connected to that tube in the countingcircuit which represent that digit. I i

The counter LTMC consists of a chain of three-electrode gas dischargetubes CCT 1, CCTZ, CCT3 and so so on up to CCTO (not shown). Thecounting circuit is of the type in which the striking of one tubeextinguishes the preceding tube and primes the next succeeding tube toenable it to strike on the next pulse. At any particular instant,therefore, only one tube is conducting. Thus when tube CCT1 isconducting the voltage developed at the junction of R41 and R42 isapplied over R43 to the priming electrode of CCTZ so that the next pulsefrom JG10 applied over C30 serves to strike the tube CCTZ. In theabsence of the priming voltage, the application of a pulse from JG10 tothe priming electrode is without effect. Thus when none of the lines arein a calling condition, each pulse from JG10 is applied to all the tubesof the counting circuit but only one tube strikes in response to aparticular pulse and in striking extinguishes the preceding tube.

When a line is in a calling condition, for instance, the line shown inthe drawing which has the tens digit, 1, the negative 30 volts acrossR40 applies a negative bias to the tube N50, thus preventing thestriking of N50 and thus of the tube CCTZ when the appropriate pulsefrom JG10 arrives. The counter is thus stopped since subsequent pulsesfrom P610 although applied to the tubes in the counter will not causethe tubes to strike owing to the absence of the priming voltage. TubeCCT1 thus remains conducting indicating that the tens digit of thecalling line is l.

A discharge tube pulse generator is located in the cathode circuit ofeach counting tube, the pulse generator associated with counting tubeCCT1 comprising resistor R46, tube N51, primary windings of T30 and T31and capacitor C32. When CCT1 is non-conducting, the 50 volts betweenearth applied through R41, R42, R46 and battery applied through T31, T30is insufficient to strike N51. When CCT1 is conducting, however, thereis volts across R41 and R42 which is applied through R46 to the tubeN51. The tube thus has 80-1-50 volts across it and strikes. Before itstrikes, however, C32 has to charge through R46. This delay is arrangedto be longer than the time during which CCT1 is conducting when thecounter is running freely, so that no pulse generator will operateduring this time. When the counter has been stopped with CCT1conducting, C32 charges up and N51 strikes, discharging C32 andtransmitting a positive pulse through T30 and T31.

The pulse from T30 is applied to the control electrodes of all tubessuch as CCL2 associated with lines having the same units digit. Aspreviously mentioned the negative 30 volts from the line circuit isapplied to the cathode of CCLZ and assuming the line SL is the only onein a calling condition, this tube will be the only one having 30 voltson the cathode and will hence be the only one to strike.

The anode of CCL2 is connected to a supply of 80 volts through R49 andwhen the tube strikes, the righthand end of R11 goes positive. Thisvoltage is applied to the P lead and serves to busy the calling line inthe final selector multiple as will be described in detail later. Inaddition the negative voltage across R40 is removed.

The pulse from T31 is applied to the sequence control circuit LSS. Inthe normal condition, the tube CCSSI in this circuit is conducting sothat its cathode is positive. This positive voltage is applied to P610to enable the latter to operate and also to one electrode of N54, theother electrode being connected to a negative potential over R48. Thetube N54 is thus conducting and the consequent positive voltage at thebottom end of R48 prevents the striking of N53 and similar pulsegenerators. The pulse from T31 extinguishes CCSSl and strikes CCSS2.When CCSSI is extinguished, the voltage developed across cathoderesistors R54 and R55 falls from a positive value to zero so that earthis now applied to the pulse generator P610 to prevent further operationthereof and also to. N54 which is thus extinguished to enable the mass?pulse generators such as N53 subsequently to operate. The striking ofCCSS2 causes a positive voltage to be developed across cathode resistorsR56 and R57 and this is applied to pulse generator PG11 to cause it tostart generating. The output from the pulse generator PG11 is connectedover ten leads to the priming electrodes of each tube in the unitscounter LUMC, the connection including a capacitor C34 and dischargetube N52. The circuit is arranged to operate in a similar manner to thetens counter, that is to say the discharge tubes such as N52 normallyconduct in response to pulses from PG11 but when tube CCL2 strikes thevoltage developed across R49 reduces the bias applied to N52 to a valuewhere the appropriate pulse from PG11 is insufiicient to strike N52.When the pulse generator starts up, therefore, the counter runs roundfrom its random starting position until tube CCUO strikes, the unitsdigit being After a suitable delay, the discharge tube N53 connected tothe cathode of CCUO strikes and the pulse developed across R47 isapplied to the control electrode of the tube CCSS3. This tube strikeswhile CCSS2 is extinguished.

The position now is that the subscriber on line 10 has lifted off hisreceiver as a result of which CCT1, CCL2, CCUl and CCSS3 have allstruck.

Connections are taken from the junction of the pair of cathode resistorsin each counting tube to a marking network in such a manner that a biasis applied in accordance with which tubes are conducting, through thenetwork to the appropriate set of cold cathode tubes in an idle linefinder which when struck connect the calling line through the linefinder to the final selector. Thus referring to the marking networkshown in Fig. 2, the leads marked 1-, 2-, 3- and 4- will be connected tothe correspondingly marked leads of the tens marking counter. Similarlythe leads marked -1, 2, 3, -4 in the marking network will be connectedto the correspondingly marked leads in the units marking counter. Assumefor example that the tubes CCT1 and CCUO are non-conducting. Then thevoltage at the junction of R41 and R42 is zero and similarly for thejunction of R52 and R53. The voltage at the point M10 in the markingnetwork is thus zero. Now assume that CCT1 and CCUO are conducting, asthey will be in the present case. The voltage at the junctions of thecathode resistors will now become positive and the circuit is soarranged in conjunction with the rectifiers in the marking network thatthe point M10 now takes up a voltage of +50. This voltage is applied tothe control electrodes of the corresponding sets of discharge tubes inall the line finders. It will be noted that the positive voltage at thejunction of the cathode resistors will also be applied to otherrectifier pairs. For instance, the voltage between R41 and R42 will beapplied to all the rectifiers connected to lead 1- while that betweenR52 and R53 will be applied to all rectifiers connected to lead -0.However the positive potential at the junction points such as 12, 13 andso on and 21, 31 and so on will be less than 50, approximately 25 andthis is below the voltage necessary for priming the line finderdischarge tubes.

The preselection of an idle line finder is effected by the allotter AL(Fig. which is in the form of a ring counter and it will be assumed thatline finder LFDRI has been allotted for use and consequently tube CCALlin the allotter is conducting. The positive voltage developed across R83is applied to the junction of R24 and R27 (Fig. 2). The voltage acrossR83 is 50. Before CCSS3 (Fig. 4) strikes, there is no voltage dropacross R51 and the component values are selected so that the junction ofR23 and R24 (Fig. 2) is at +25 volts and V remains non-conducting, thecathode voltage of V10 being greater than +25 volts, but less than +50volts. When CCSS3 conducts as described above, this voltage is increasedto +50 and V10 conducts. However, the buildup of the control gridvoltageof V10 is comparatively slow so that no pulse is obtained in the V10output. As-

sociated with V10 is a continuously operating pulse generator comprisingneon tube N20, resistors R21, R20 and capacitor C20. This pulsegenerator feeds negative-going pulses to the control grid of V10 throughC21 and when V10 is rendered conducting as mentioned above, the pulsesfrom the pulse generator appear in the anode circuit of V10 aspositive-going pulses, the negative-going pulses on the grid being ofsufficient amplitude to cut-off V10. These positive-going pulses are fedinto the grid circuits of every tube in the associated line finder butare of insuificient amplitude to cause the tubes to strike unless a biasis applied from the marking network as described above. In the presentcase therefore only the tubes shown i. e. those leading to line 10 willstrike to extend the line to the final selector via the two-wayamplifier AMP.

When tube CCLF3 in the private lead strikes a negative pulse isdeveloped across the primary winding of the transformer T30 which iscommon to all private anodes of the finder switch. This pulse isinverted and fed as a positive pulse from the secondary winding into thegrid of CCBU. Since CCALl (Fig. 5) is conducting, CCBU is primed throughR27 and therefore strikes. R25 and R26 now have volts dropped acrossthem, and this cuts off the pulse generator due to the connectionthereof to the cathode of CCBU. R26 has 50 volts across it and this isfed through R28 to the allotter step-on control ASC (Fig. 5). Aconnection is also made to the lead from the cathode of CCALl via R84.Thus, if CCBU is not conducting and CCALI is conducting, the junction ofR84 and R28 is at +25 volts and MR55 and V26 are not affected. Whenhowever, CCBU conducts the junction of R84 and R28 will be at +25 voltsand MR55 conducts. building up a positive voltage across R85. Valve V26conducts and cuts off V27, which allows pulse generator N42 to operate.This drives the allotter to the next position. If the tube correspondingto CCBU in the next finder is conducting V26 conducts again, since R85has a positive voltage across it due to MR54 conducting and N42 pulsesagain. N42 will drive the allotter until a free line finder is found, i.e. until a tube corresponding to CCBU which is not conducting is found.The allotter will then come to rest and that particular finder will bepreselected.

While a free line finder is being found V28 (Fig. 5) will be conducting.When a free finder is located, V28 ceases to conduct abruptly since N43and N44 are extinguished. A positive pulse will thus be transmitted fromthe anode of V28 to the sequence switch LSS, driving the counter fromCCSS3 to CCSSl. This resets the control and marking circuit, and LTMCdrives round until another call comes up. Further when CCLF3 in theprivate lead strikes, the right-hand end of R11 goes more positive, thevalue of R11 being high compared with the resistance of L10. Thisincreased positive potential is applied to the cathode of CCLZ which isthus extinguished.

Consideration will now be given to the operation of the electronicequivalents of the A, B and C relays of the and V23 are normallynon-conducting while CCC is conv ducting. Tube 34 is thusnon-conducting.

Alternating current having a frequency considerably greater than thedial frequency is fed over lead 10 (Fig. 2) and condenser C10 to thetube N10 and to L10. Owing to the negative voltage developed across L10when the subscriber lifts his receiver, the negative half-wave of thealternating current causes N10 to strike so that pulses are fed throughC10 to the private lead. These pulses are, however, ineffective untilthe line finder has extended the subscribers line to the finalselector.When this occurs, tube CCLF3 in the finder conducts and the pulses onthe private lead are transmitted through CCLF3 to the primary winding ofthe tuned transformer T30. The pulses are stepped up and filtered in thesecondary winding of T30 and are fed to the input of the tube V20 (Fig.5) which is arranged to operate as a limiter-amplifier and is providedwith a transformer T50 in the anode circuit. The output from thesecondary winding of T50 is fed to a voltage-doubler circuit of knowntype, the negative D. C. output of which is fed via R61 to the controlgrid of V21. It will be noted that one side of the voltagedoublercircuit is negatively biased for pulse-shaping purposes. The negativeoutput from the voltage-doubler is applied to V21 and causes the tube tobe cut oil, thus allowing N30 to strike. This causes V22 to conduct thusextinguishing CCB but N31 can still not strike nor can N32 or N33.

Dial tone is returned to the calling subscriber in a manner to bedescribed later and he operates the dial switch in accordance with therequired number. On the first break over the loop, the negativepotential across L is removed and consequently the alternating currentfed over lead 10 is without efiect on the tube N10. No pulses aretherefore fed over the private lead to the limiter-amplifier and theoutput of the voltage-doubler goes positive to such an extent as toenable V21 to conduct again. Tube N30 is thus extinguished and V22 iscut off. It should be noted that CO3 does not strike at this time sincethe 25 volt priming bolts applied over R65 is not high enough. Tubes N32and N33 strike but the striking of N31 is delayed by the provision ofcapacitor C41 and the duration of the delay is arranged to be greaterthan the duration of the longest break pulse likely to be encountered.Tubes N32 and N33 in striking cause V23 to conduct thus extinguishingCCC. At the end of the first break period, the alternating current willbe again transmitted to the limiter-amplifier whereupon thenegative-going D. C. output from the voltage doubler will again out ofiV21. Tube N30 again strikes, allowing V22 to conduct and extinguishingN32 and N33 so that V23 is again cut off. Tube N34 does not, however,strike at this time since, similarly to N31, its striking is delayed bycondenser C43 for a period longer than the duration of the longest make.

A similar operation occurs for each break pulse in the train. At the endof the impulse train, as V23 is cut off for a longer period than theduration of the longest make, N34 will eventually strike, causing apositive pulse to be developed across R72 whereupon CCC again strikesand N34 is extinguished. This positive pulse is also fed to the finalselector sequence switch FSS as described later.

It will be seen from this that tube V22 follows the impulses transmittedby the calling subscribers dial switch in that it is non-conductingduring breaks. The positivegoing pulses developed in the anode circuitare taken from the junction of the anode resistors R62 and R63 tocircuits which effect the setting of the final selector in a manner tobe described later. It will also be noted that tube N31 does not strikeduring impulsing and it is this that gives the associated circuit thecharacter of the B relay. This will be better appreciated from thedescription of the release of the connection when it will be seen thatN31 strikes to initiate this release when the calling party hangs up.Finally it will be seen that the circuit associated with tube N34 hasthe characteristic of a C relay since tube N34 strikes at the end of animpulse train to perform a switching operation as described later.

When the final selector is taken into use, tube MA in the final selectorsequence switch PS8 is conducting. The positive voltage across R74 isthus applied to N21 in the final selector tens marking circuit ,FIMC.This tube thus strikes and consequently the pulses from the junction ofR62, R63 which follow the dial impulses are fed to FT.MC, and appear aspositive pulses across R30. As previously mentioned FTMC operates in asimilar manner to LTMC and thus takes up a position corresponding to thenumber of pulses in the first train. At the end of the first train N34strikes as previously described and the positive pulse developed acrossR72 is applied via C44 to cause MB to strike in PS8 and consequently toextinguish MA. When MA is extinguished, N21 in FT MC is extinguished butthe striking of MB causes N22 in FUMC to strike. The second train ofpulses is thus applied to the units marking circuit. At the end of thesecond train N34 strikes again, extinguishes MB in FSS and strikesMC.

In each of the cathode circuits of the units counter are two pulsegenerators which will be referred to as the A and B generators. The timeconstant of A is greater than a break period so that while the counteris being operated, no A pulse generator is operated. The B pulsecounters are controlled as described later. When the counter stops with,say, tube CCU(1) conducting, the associated A pulse generator comprisingN23 strikes after a delay. A positive pulse is thus fed over lead UMAIto all the UMA leads of the private tubes of the sets of discharge tubescorresponding to the particular units digit. Now assume that the tenscounter has been stopped with tube CCT(2) conducting, the positivevoltage at the cathode will be applied over lead TM2 to the gridresistors of all the sets of tubes corresponding to the particular tensdigit. Hence only one private tube has applied to it the priming voltagefrom the tens counter and the striking pulse from the units counter.Assuming that the line represented by the two digits is idle, theprivate tube, say, CCFS3 strikes and in striking passes a positive pulsethrough T51 to strike HAA, HAA being primed by the positive voltagedeveloped across the cathode resistor of N39 and N40 which are normallyconducting.

The tubes HAA and HAB are analogous to the H relay in the finalselector. Together they form a bi-stable element of which HAB isnormally conducting. In this condition a voltage is developed across Rand accordingly MR50 conducts and the consequent current flow causes avoltage to bedeveloped across R82. This voltage will be positive withrespect to negative battery and will be applied over lead 27 to all theB generators, of which two only, namely N24 and N26, are shown in thedrawing. This positive bias applied to the B generators prevents theiroperation when the associated counting tube strikes. However, when HAAstrikes, current ceases to fiow through R80 and R82 and the full voltageof the negative battery is applied to the B generator, in this case N24,which thereupon operates and over lead UMBI applies a positive pulse tothe remaining tubes of the selected set. Tubes CCFSI, CCFS2 and CCFS4,therefore, strike, It will thus be seen that access to a required lineis obtained by applying a pulse to the P tube of the set of tubes havingaccess to the desired outlet and if this tube strikes, indicating thatthe line is available, a pulse is then applied to cause the remainingtubes of the set to strike.

When tube MC of the final selector sequence switch strikes as explainedabove, a positive pulse is applied to N41 (Fig. 5) which together withtubes CCE, N39 and N40 forms the electronic equivalent of the E relay.It is, however, arranged, by means of the parallelconnected capacitor,that the striking of N41 in response to the positive pulse is delayedfor slightly longer than the time taken for N23 to strike, HAA to strikeand N24 to strike. When N41 eventually strikes, it causes CCE to strikeand the positive voltage developed across the cathode resistor isapplied to lead 26 and through MRSl to lead 27. This positive voltageextinguishes 9th ubes N2 and N 4, while QCE in con q ns,

ca'u'sesN39 and to be extinguished thereby remov ing the priming voltagefrom HAA.

As a further result of the striking of HAA, the positive voltage acrossR81 is applied over R78 as a priming voltage to tube CCF which isanalogous to the F relay in a final selector.

When tube CCFS4 strikes, current flows over the D lead and a resistorsuch as R86 (Fig. 2) in the wanted subscribers line circuit. Thepositive voltage developed across this resistor is applied via resistorR87 to strike the tube, such as CCLl, which is individual to thesubscribers line circuit. When this tube strikes, ringing current isapplied from source RC through transformer T and the tube to the wantedline.

When the called subscriber removes his receiver, alternating current isinjected into the private lead as in the case of a calling subscriber.This is extended over the final selector multiple to the private lead inthe final selector and thence through tube CCFS3 to T51. Here thealternating current is amplified in V24, limited and rectified to give anegative D. C. bias to V25. This tube is normally conducting but is cutoff by the negative bias to allow N36 and N37 to strike. In striking N37applies a positive pulse to CCF which, being primed when HAA strikes,now strikes and primes N38. The alternating current applied to N38 nowpasses through the tube and the positive voltage developed across R79 onpositive half-waves serves to strike HAB and to ex tinguish HAA.

In addition when the called subscriber replies, a negative voltage isdeveloped across the impedance corresponding to L10. This extinguishesthe tube corresponding to CCLll to disconnect ringing. Finally when CCFconducts, a large negative potential is developed across L50 which issuflicient to extinguish tube CCFS4 to prevent further striking of thetube corresponding to CCLI.

The operation of the circuit in the case where the wanted line is busywill now be considered. The tube CCLF3 or a corresponding tube inanother finder will be conducting at this time and the current flowthrough the resistor corresponding to R11 causes the right-hand end ofR11 to go approximately 70 volts positive. This voltage is applied tothe P lead in the final selector multiple and thus to the cathodes ofall the tubes such as CCFS3. Consequently the tube CCFS3 in the presentexample, will fail to strike in response to the application of thebiasing voltage and the operating pulse. Tube HAA is therefore notstruck and HAB remains conducting so the full negative voltage is notapplied to the B tubes which are therefore prevented from striking. TubeN41 however, conducts after its delay since it is controlled by tube MCof the sequence switch. Consequently CCE also strikes to exert theabove-described control over leads 26 and 27 and to extinguish tubes N39and N40. The tone control circuit TSC is arranged in such a manner thatwith HAA conducting (wanted line idle), ring back tone is fed to thecalling subscriber when CCE strikes while with HAB conducting (wantedline busy), busy tone is fed to the calling subscriber. Referring to thetone circuit, the leads 21, 22 and 23 are connected to a tapping on thecathode resistor of HAB, a tapping on the cathode resistor of HAA andthe cathode of CCE respectively. Now CCE strikes whether the wanted lineis busy or idle and thus a positive potential with respect to negativebattery is applied to lead 23 in both cases. If the line is idle HAAstrikes and a positive potential is applied to lead 22 while negativebattery potential is applied to lead 21 since HAB is extinguished.Further lead 25 will be connected to earth potential since CCF does notstrike until the called subscriber replies. Hence the junction betweenrectifiers MR30 and MR31 in the tone circuit will be at a positivepotential and the positive side of MR34 will be at earth potential sothat MR34 will conduct and ring-back tone over lead RT will betransmitted to the 10 calling party over transformer T21. subscriberreplies, CCF conducts and a positive potential is applied to lead 25,this potential having a higher value than that at the junction of MR andMR31. Rectifier MR34 thus ceases to conduct and the transmission ofring-back tone to the calling subscriber is prevented. It will be notedthat at this time the junction between MR32 and MR33 will be at apotential between that of the negative battery and the positivepotential at the cathode of CCE while the positive side of MR35 is atearth potential. It is arranged that the potential between MR32 and MR33is below earth so that MR35 does not conduct and the transmission ofbusy tone to the calling party is prevented. wanted line is busy,conditions in the tone circuit are reversed and MR35 conducts while MR34is block. Busy tone is thus returned to the calling subscriber.

With regard to the transmission of dial tone, it should be explainedthat when a line finder switches to a calling line, the sequence switchin the associated final selector will have MA conducting. A positivepotential having a value greater than volts will thus be applied to lead24 so that MR36 will be conducting and dial tone will be fed to thecalling line over T21 as soon as the line finder switches through. Whenthe first train of impulses has been received, MA is extinguished andlead 24 is then earthed. Rectifier MR36 ceases to conduct and thefurther transmission of dial tone is prevented.

At the end of the call, the calling and called parties both replacetheir receivers and accordingly current is disconnected from the privatelead extending from both line circuits. As regards the calling line thisenables tube V21 to conduct again thus causing N30 to be extinguished.As regards the called line, V25 again conducts and N36 is extinguished.When both N30 and N36 are extinguished, V22 is cut off and N31 strikes.The consequent positive potential developed over R67 is applied to ZAwhich strikes and ceases the operation of relay Z. Relay Z at contactsZ1 disconnects the H. T. supply from all points marked X in the drawing.This extinguishes the tubes in the speaking and private leads and thetubes in the counting circuits FTMC and FUMC. It also extinguishes theconducting tube in the final selector sequence switch PS8 and the tubeZC, while is normally conducting. Further it extinguishes tube CCE (Fig.6) in the final selector and tube CCBU in the line finder thus makingthe line finder available to the allotter. Finally it disconnects the H.T. supply from V20 and V24 and from tube CCE which is thus extinguished.

It will also be noted that the positive potential across R67 causes apulse to be applied to CCB which strikes and extinguishes N31.

The tube ZA forms a bi-stable element with ZB which is normallyconducting. When ZA strikes, ZB is extinguished thus enabling N35 tostrike after a delay. When N35 strikes, ZB again strikes and ZA isextinguished. Relay Z thus releases to reconnect the H. T. supply. Thereconnection of the H. T. supply causes ZC to strike and apply a pulseto the normal position tubes in the counters FUMC and FTMC and in thesequence switch FSS.

All the circuits are now in readiness to accept another call. Release inthe case where the called line is busy takes place in a similar mannerwhen the calling subscriber replaces his receiver and it is thought thatno detailed description thereof is necessary.

It will be appreciated that various modifications may be made to thecircuits described without departing from the scope of the invention asset forth in the claims. For instance, the sets of discharge tubes havebeen shown as each comprising individual tubes but it is possible that anumber of tubes may be grouped together in one envelope. Further whilethe system shown is a two-coordinate arrangement only, the capacity of.the system could be enlarged by employing a three-coordinate arrange-'When the called If the.

ment. Again it will be appreciated that although no group selectingfeature has been disclosed, it would be a simple matter to adapt thefinal selector to operate as a group selector. For this purpose thecounter FUMC would be operated from a pulse source and a test for theidle or busy condition of the outlets of the selected group would bemade after each step of the counter.

It will be noted that the discharge tubes shown in the drawing areeither of the two or three electrode type cold cathode tube. While theseare preferable it is to be understood that the invention is not limitedthereto but other types may be used if desired.

We claim:

1. In a telecommunication system, a switching arrangement comprising aplurality of sets of gaseous discharge tubes arranged in groups, inputpaths to said switching arrangement, output paths from said switchingarrangement, each input path communicating with an output path over thedischarge paths of one set of gaseous discharge tubes, control means forapplying a first potential to all the sets of gaseous discharge tubes ina group, and control means for applying a second potential to one set ofgaseous discharge tubes in each group, each set of gaseous dischargetubes striking only in response to the concurrent application of bothsaid first and second potentials thereto to complete a connectionbetween an input path and an output path.

2. In a telecommunication system, the arrangement claimed in claim 1 inwhich the potential applied by one of said control means is a steadypotential and the potential applied by the other of said control meansis of pulse form.

3. In a telecommunication system, the arrangement claimed in claim 1wherein the first mentioned control means is a non-numerical controlmeans and wherein the second mentioned control means is a numericalcontrol means.

4. In a telecommunication system, the arrangement as claimed in claim 1wherein the first and second mentioned control means are each numericalcontrol means.

5. In a telephone system, a plurality of subscribers lines, switchingmeans comprising a plurality of sets of gaseous discharge tubes arrangedin groups, each discharge tube having at least two electrodes, circuitmeans including the discharge paths of said tubes and constitutingconversational and control signal paths between said subscribers linesand subsequent equipment, means for determining the number of a callingsubscribers line and for applying a bias potential to correspondingelectrodes of a set of discharge tubes in each group in accordance withthe number of the calling line and control means for applying apotential of pulse form to corresponding electrodes of all the sets ofdischarge tubes in a group, said discharge tubes striking only upon theconcurrent application of said bias potential and said pulse-formpotential thereto to complete a connection between said calling line andsaid subsequent equipment.

6. In a telephone system, the arrangements claimed in claim 5 andcomprising a plurality of counting circuits each comprising a ring ofgaseous discharge tubes, means for applying pulses of potential to saidcounting circuits successively to cause the tubes of each ring to strikein sequence and means for stopping the striking sequence of the tubes insaid counting circuit at points representative of the number of thecalling line.

7. In a telephone system, the arrangements claimed in claim 6 andcomprising, in addition, a pulse generator, means for applying pulsesfrom said pulse generator to one of said counting circuits to effect thecontinuous operation thereof, leads extending from subscribers lines andmultipled together in accordance with one digit of the numericaldesignations of the lines, means responsive to the calling condition ofa line for applying a voltage to a particular oneofr" said leads andmeans responsive to the application of said voltage to said lead forcontrolling said first mentioned means whereby the application offurther pulses to said counting circuit is prevented.

8. In a telephone system, the arrangements claimed in claim 7comprising, in addition, an output circuit for each stage of said onecounting circuit, means for delaying the transmission of a pulse throughan output circuit whereby an output pulse is obtained only when theoperation of said one counter is stopped further leads extending fromthe subscribers lines, which leads are multipled together in accordancewith a further digit of the numerical designations of the lines, asecond pulse generator for applying pulses to a second counting circuit,means responsive to an output pulse from said one counting circuit forextending said leads to control said second pulse generator and meansfor initiating the operation of said second pulse generator.

9. In a telephone system, the arrangements claimed in claim 8comprising, in addition, a co-ordinate marking network and meansresponsive to the setting of the counting circuits in accordance withthe calling subscribers number and operative to feed a potential throughsaid co-ordinate marking network to corresponding electrodes of a set ofdischarge tubes in each group.

10. In a telephone system the arrangements claimed in claim 9 in which asequence switching device comprising a plurality of serially arrangeddischarge tubes is provided for controlling the sequential operation ofsaid counting circuits.

11. In a telephone system, the arrangements as claimed in claim 10 inwhich means are provided responsive to the sequence switching devicereaching the final point in its switching sequence and operative toapply a voltage to a preselected group of sets of discharge tubes tocause a pulse to be applied to corresponding electrodes of all said setsof discharge tubes.

12. A switching arrangement as claimed in claim 11 including allottingarrangements comprising a plurality of serially-arranged discharge tubesoperative to preselect ditferent groups in dependence upon which tube inthe series is struck and responsive to the operation of one set ofdischarge tubes to transfer the conducting condition from one tube tothe next tube in the series so as to pro-select another group and tocause said sequence switching device to switch to the first point in itsswitching sequence, said counting circuits being responsive to theswitching of said sequence switch to reset to normal.

13. In a telephone system, a plurality of subscribers lines, switchingmeans comprising a plurality of sets of gaseous discharge tubes arrangedin groups, each discharge tube having at least two electrodes, circuitmeans including the discharge paths of said tubes and constitutingconversational and control signal paths for completing connections to acalled subscribers line, means for storing the penultimate and finaldigits of a called subscribers number and serving to apply a biaspotential to corresponding electrodes of all the sets of discharge tubesin a particular group as determined by one of said digits and to apply apulse potential to particular corresponding electrodes of correspondingsets of discharge tubes in all groups as determined by the other of saiddigits, said sets of discharge tubes striking only in response to theconcurrent application of said bias potential and said pulse potentialthereto to complete a connection to a called subscribers line.

14. In a telephone system, the arrangements claimed in claim 13 andcomprising, in addition, first and second counting circuits for storingsaid digits of the called subscribers number each said counting circuitconsisting of a ring of discharge tubes and a sequence switching devicecomprising a plurality of serially-arranged discharge tubes for routingthe penultimate and final digits to the appropriate counting circuit.

15. In a telephone system the arrangements claimed in-claim .1 and c mrisin i a d n. n mp s I3 sponsive device, parallel connected outputcircuits for said impulse-responsive device connected to said countingcircuits, and a blocking discharge tube in each of said output circuits,said discharge tubes being rendered conducting sucessively under thecontrol of said sequence switching device.

16. In a telephone system, the arrangements claimed in claim 14 whereinsaid impulse-responsive device comprises a thermionic tube, means tocause said tube to become conductive in response to the connection of acalling line through to the switching means and means are providedresponsive to the break periods of an impulse train from the callingsubscribers dial switch to render said tube non-conducting.

17. In a telephone system, the arrangements claimed in claim 16including, in addition, a source of alternating current associated witheach subscribers line circuit, means responsive to the removal of acalling subscribers handset for causing transmission of alternatingcurrent from said source over the private conductor of the callingsubscribers line circuit, means responsive to the transmission ofalternating current over said conductor to cause said thermionic tube toconduct and responsive to the operation of said dial switch to rendersaid tube nonconducting.

18. In a telephone system, the arrangements claimed in claim 17 andcomprising, in addition, a second thermionic tube, a trigger circuitbetween said first and second thermionic tubes whereby said thermionictubes are always in complementary operating conditions, athree-electrode discharge tube connected between the anode and cathodeof said second thermionic tube, a two-electrode discharge tube connectedbetween the anode and priming electrode of said three-electrode tube andmeans for delaying the striking of said two-electrode tube whereby it isonly struck in response to the non-conduction of said second thermionictube during the interdigital pause while said three-electrode tube isnon-conducting, said twoelectrode tube in striking serving to operatethe sequence switching and to strike said three-electrode tube therebybeing itself extinguished.

19. In a telephone system the arrangements claimed in claim 18 andcomprising, in addition, a three-electrode discharge tube connectedacross said second thermionic tube, a two-electrode tube connectedbetween the anode and priming electrode of said three-electrode tube,means for delaying the striking of said two-electrode tube whereby it isonly struck in response to prolonged non-conduction of said secondthermionic tube while said three-electrode tube is non-conducting, saidtwo-electrode tube in striking serving to initiate the release of theconnection and to strike said three-electrode tube whereby it is itselfextinguished.

20. In a telephone system, the arrangements claimed in claim 17 in whichsaid second thermionic tube is responsive to the flow of alternatingcurrent over the private leads extending from both the calling andcalled parties line circuits to remain conducting until the cessation ofsuch flow.

21. In a telephone system, the arrangements claimed in claim 14 andcomprising, in addition, means responsive to the registration of saidtwo digits on said counting circuits for causing a test to be made ofthe idle or busy condition of the called line.

22. In a telephone system, the arrangements claimed in claim 21 andcomprising, in addition, a bi-stable element consisting of a pair ofdischarge tubes responsive to the idle condition of the called line tochange from one stable condition to the other stable condition.

23. In a telephone system, the arrangements claimed in claim 22including, in addition, busy tone transmitting means, ring-back tonetransmitting means and a discharge tube operative in conjunction withsaid bistable element in said one stable condition to connect said meansfor transmitting busy tone to the calling partys line and in conjunctionwith said bi-stabl e element in the other said stable condition toconnect said means for transmitting ring-back tone to the calling partysline.

24. In a telephone system, the arrangements claimed in claim 23 whereinsaid discharge tube is operative in conjunction with said bi-stableelement in said one stable condition positively to prevent the strikingof the discharge tubes in said switching means for completingconnections to the called party.

25. In a telephone system, the arrangements claimed in claim 23including means responsive to the removal of the called partys handsetto transmit alternating current over the private lead to cause thestriking of a further discharge tube, said further discharge tube beingoperative to prevent transmission of ring-back tone to the calling partyand to restore said bi-stable element to said one stable condition.

26. In a telephone system, the arrangements claimed in claim 23 andcomprising a switching network consisting of a plurality of metalrectifiers and biasing means for said switching network dependent uponsaid bi-stable element.

27. In a telephone system, the arrangements claimed in claim 21 in whichsaid bi-stable element is operative in the said other stable conditionto control the application of potential to cause the striking of thedischarge tubes in the switching means which complete the connection tothe called party over conversational leads and the private lead.

28. A telephone system comprising a plurality of subscribers lines, afirst plurality of sets of gaseous discharge tubes, a second pluralityof sets of gaseous discharge tubes, means for successively triggeringone set of said first plurality of sets and one set of said secondplurality of sets to complete a connection between a calling and acalled line, means responsive to the triggering of said one set of saidfirst plurality of sets for preventing thereafter the triggering of anyof the sets of said second plurality connected to said calling line andmeans responsive to the triggering of said one set of said secondplurality of sets for preventing thereafter the triggering of any of thesets of said first plurality connected to said called line.

29. A telephone system, as claimed in claim 28 in which in each set ofdischarge tubes of said first and second plurality of sets one dischargetube is responsive to the operation of said triggering means and meansare provided responsive to the striking of said one tube to apply a biaspotential to an electrode of each of the remaining discharge tubes insaid one set.

30. An electronic relay comprisinga thermionic tube, an anode circuittherefor, a gaseous discharge tube having at least two electrodesbetween which the discharge takes place, connecting means between saidanode circuit and one of said electrodes, a source of negative potentialand connecting means between said source of negative potential and thesecond of said electrodes whereby said discharge tube strikes only whensaid thermionic tube is cut oil.

31. An electronic relay as claimed in claim 30 in which said dischargetube has three electrodes and a second discharge tube is connectedacross the anode and priming electrode of said three electrode tube,said priming electrode being connected to a source of priming voltage.

32. An electronic relay as claimed in claim 31 in which a capacitor isconnected across said second discharge tube to delay the strikingthereof.

33. In a telecommunication or like system, a circuit arrangementemploying two electronic relays as claimed in claim 31 in which theanode of the thermionic tube of the first electronic relay is connectedto the control grid of the thermionic tube of the second electronicrelay over a discharge tube and a source of negative potential is con-15 nected to said control grid, the striking of the second gaseousdischarge tubes of each relay being delayed by different amounts.

34. A telephone system comprising a plurality of subscribers lines, aplurality of line circuits each connected to one of said lines, aplurality of sets of gaseous discharge tubes through whichconversational and control paths are extended on the striking of thetubes of a set, a source of alternating current in each line circuit, adigit storage device, means responsive to the striking of one of saidsets of tubes for connecting one of said line circuits to said digitstorage device and for completing a path from the source of alternatingcurrent in said one line circuit through one tube of said set to saiddigit storage device, means responsive to the transmission of impulsesrepresenting a digit over the subscribers line connected to said oneline circuit for intermittently interrupting the transmission ofalternating current over said path to enable the digit to be stored insaid digit storage device and means responsive to prolonged interruptionof said alternating current to release the connection set up from or toa o e ine c c it- 35. In a telephone system, a switching stagecomprising a plurality of sets of gaseous discharge tubes, means forstriking a set of tubes to complete conversational and control pathsthrough the stage, a source of alternating current, a receiving devicefor said alternating current, means responsive to the striking of a setof tubes in the stage for transmitting alternating current from saidsource to said receiving device over a control path, means forinterrupting the flow of alternating current over said control path andmeans in said receiving device operative in response to the interruptionofalternating current flow over said control ,path to effect control andsupervisory functions.

36. 'In a telephone system, the arrangements claimed in claim 35 inwhich there is provided timing means responsive to interruptions in theflow of alternating current of greater than a predetermined duration toextinguish said set of tubes.

37. In a telephone system the arrangements claimed in claim 36 andincluding a calling line circuit, a called line circuit and at least oneof said switching stages extending a call between said calling line andsaid called line, means in said calling line circuit responsive to thetermination of said call at the calling end and jointly efiective withmeans in the called line circuit responsive to determination of saidcall at the called end to prevent the transmission of said alternatingcurrent over said control path whereby to render said timing meanselfective to extinguish the set oftubes in said switching stage.

References Cited in the file of this patent UNITED STATES PATENTS

